Technical Field of the Disclosure
The present embodiment relates in general to a method and apparatus for generating hydrogen. More specifically, the present disclosure relates to a method and apparatus for producing hydrogen and aluminum oxide from solid aluminum using water splitting techniques.
Description of the Related Art
Hydrogen can be generated by a variety of methods. Hydrogen can be generated by natural gas reforming, electrolysis, thermochemical reaction and photo catalytic methodologies. These methodologies produce carbon dioxide as a by-product, which requires a substantially large amount of electrical energy which can be expensive and have a large, negative environmental impact. It requires solar energy with temperatures exceeding 1000 degrees Celsius; it requires highly corrosive reactants and/or products; and often requires expensive reagents, complex nanostructured solids, and/or sacrificial oxidants or reductants other than water.
A number of variants of the water split reaction used to produce hydrogen have been devised to overcome these problems. The water split reaction contemplates a fuel for splitting water into hydrogen and an oxide. In these reactions aluminum is used to generate hydrogen from water. Commonly, aluminum oxide compounds can be produced from bauxite ores by Bayer's process. In the water splitting process, the hydrogen is released as a gas and the oxygen combines with the aluminum to form the aluminum oxide compounds. The aluminum oxide compounds are produced as a protective oxide layer on the aluminum in contact with water at ambient temperature.
Aluminum has a tendency to be self-protecting by forming the aluminum oxide that inhibits reactions required for the formation of hydrogen and thus in some cases is difficult, if not, impossible to use on a long term basis. Therefore, it has been accepted by those skilled in the art that the use of aluminum in a reaction with water to generate hydrogen gas requires that the protective oxide layer is efficiently and continuously removed, and that the reaction is kept at an elevated temperature.
In one prior art reference, U.S. Pat. No. 4,358,291, the inventors disclosed that if aluminum (Al) is dissolved in a liquid solution of gallium (Ga) or a liquid mixture of Ga and indium (In) at or near room temperature, and brought into contact with water, the Al in the liquid solution at the water interface would split water molecules (H2O) into hydrogen gas, alumina (Al2O3), and generate heat. This reaction will proceed until all elemental Al in the liquid solution is converted to alumina. The solid aluminum (Al) will dissolve in dry, air exposed liquid melts of gallium (Ga), Ga-indium (Ga—In), or Ga—In-tin (Ga—In—Sn) at or near room temperature up to the solubility limit, about 2-3 weight percent Al.
When Al is dissolved in liquid Ga and is brought into contact with water, the Al that is dissolved in the Ga is no longer passivated by its oxide. The result is that the Al at the water-Ga metal solution interface will split water into hydrogen gas, heat and aluminum hydroxide. Thus, the reaction presents a technological barrier to continuously resupply depleted Ga melts with more Al in the presence of water. This barrier to further dissolution in the Ga melts is the result of an interface layer of water that formed between Ga surface and the floating Al surface in the presence of excess water. This water forms a more impervious aluminum oxide layer than that which forms on an Al surface exposed just to the air. As a result Ga is prevented from making direct contact with metallic Al under the oxide layer. However, it was subsequently discovered that, in the presence of excess water, once the Al that was dissolved in the Ga solution was depleted, and solid Al was floated on the Ga liquid surface, it would not continue to dissolve to replace the depleted Al.
In addition, if the liquid solution containing Al is cooled to the point of freezing into a solid solution, very little reaction will occur. This is because unlike the case for liquid solutions, where the Al atoms can continuously diffuse to water-solution interface and react until the Al has all reacted, Al atoms in the frozen solution cannot move to the interface. Hence, only those Al atoms at the frozen solution surface can react with water. Once the Al atoms at the frozen solution surface react with water, the reaction stops.
Another drawback of this approach is that if the Al that is dissolved in the liquid solution in a dry environment and reacted to completion in the presence of excess water, the liquid solution is now under saturated with respect to Al. This means that the liquid could theoretically be saturated with additional Al. When a solid piece of Al (whose density is less than liquid Ga) is floated on top of an under saturated liquid of Ga in the presence of excess water, the solid piece of Al will not dissolve into an under saturated Ga, Ga—In, or Ga—In-tin (Sn) liquid at or near room temperature. Further, the solid Al does not dissolve in under saturated liquid Ga in the presence of water due to the fact that there is a layer of water between the liquid Ga and the solid Al that forms a barrier layer of alumina that is thicker than the alumina layer that forms between Ga and Al in air. Attempts have been made to find other methods to cope with these problems. One method is to heat a mixture of solid Al and Ga (or Ga—In or Ga—In—Sn) in an inert container above the melting point of Al, and then return the melt mixture back to the room temperature. However, this method requires the use of crucible materials that will not react with Al melts and to empirically find optimal cooling rates and composition that will render the mixture suitable for practical applications.
Another existing prior art reference, U.S. Pat. No. 8,080,233, discloses a fuel for splitting water into hydrogen and an oxide component comprises a substantially solid pellet formed from a solid-like mixture of a solid-state source material capable of oxidizing in water to form hydrogen and a passivation surface layer of the oxide component, and a passivation preventing agent that is substantially inert to water in an effective amount to prevent passivation of the solid-state material during oxidation. The pellets are brought into contact with an alloy of the passivation preventing agent having a melting point temperature below that of the solid-like mixture to initiate the hydrogen-producing reaction at a lower temperature. However, the fuel does not include any means to prevent the aluminum pellets from coming into direct contact with water to provide continuous dissolution of the aluminum pellets in passivation preventing agent in the presence of excess water.
In light of the foregoing, there is a need for a method and apparatus for producing hydrogen and aluminum oxides from aluminum using water splitting techniques that avoids the inherent problems with current technologies. In fulfillment of this need, the inventors have discovered that Al will not readily dissolve in Ga melts if the Al is simultaneously in contact with both the Ga melt and in contact with water. They have subsequently discovered that if the Al is immersed in Ga without contact with water, the Al will continue to dissolve even though water may be in contact with Ga and Ga with dissolved Al. The inventive method and apparatus described herein continuously dissolves solid-state Al or other liquid metals into a liquid Ga and its alloys in the presence of excess water at or near room temperature to enable the continuous generation of hydrogen gas and the continuous production of economically important oxides of Al or other liquid metals. The method and apparatus includes a means for splitting water in which a solid-state Al is submerged in a liquid Ga and does not make contact with water. Furthermore, the method and apparatus consists of a solid-state Al submerged below a liquid Ga interface so that the solid-state material dissolves in the liquid Ga. Finally, the method and apparatus removes the barrier layer by continuously dissolving Al into liquid Ga in the present of excess water.